1. Field of the Invention
The present invention is generally directed to a method of making chalcogenide glasses, including rare-earth doped chalcogenide glasses, and the materials produced by such method.
2. Description of the Prior Art
To date, the typical way to melt a chalcogenide glass is to heat the elemental precursors in an evacuated and sealed quartz ampoule. The furnace is a rocking furnace which assists in mixing of the melt (FIG. 1). After several hours of rocking at elevated temperature, the furnace is placed at an angle of about 45 degrees and the ampoule containing the melt is pulled out, held vertical for several seconds (FIG. 2), then immersed in water to quench the melt. The problem is that when the ampoule is set from a 45 degree angle to a 90 degree angle, the top of the glass melt near the meniscus undergoes turbulent viscous flow. The glass melt surface area of the 45 degree angle (SA)45 is estimated to be more than 3 times that of the glass melt surface area of the 90 degree angle (SA)90 (FIGS. 2A and 2B). When the ampoule is quenched in water, that unstable and viscous state near the top of the glass melt is frozen in place. This leads to the typical refractive index perturbations observed in these glasses. In addition, the melts may interact with the quartz ampoule.
During quenching, the heat loss conduction mechanism also gives rise to a large meniscus (FIG. 3). FIG. 3A shows the meniscus of the glass melt at 400° C. just before quenching in water. FIG. 3B shows the formation of the meniscus during the quenching in water. When the ampoule is submerged in water, the glass melt along the inner wall of the ampoule freezes, including the bottom region of the ampoule. Heat is transferred from a higher temperature glass melt center region through the ampoule/glass melt interface and into the water. Formation of the meniscus continues as the temperature drops due to shrinking of the glass melt via heat conduction loss mechanism through the ampoule/glass melt interface. This conventional quenching process leads to a large meniscus and, therefore, lower yield of useable glass. From a commercial perspective, this increases the cost of the glass.
During submersion in water, the melt quenches rapidly and leads to rapid pull away of the glass all at once from the quartz, leading to a powerful shock wave which causes cracking of the chalcogenide glass. This can be manifested as micro-cracking in the glass or can sometimes lead to catastrophic failure of the glass. This problem has prevented the fabrication of rare-earth doped chalcogenide glass fiber lasers.
Further, metal oxides and hydrides have strong absorption bands in the infrared wavelength region, which tend to lower the phonon energy of the glass thereby reducing radiative lifetimes of rare earth ions. Therefore, oxygen and hydrogen impurities will affect the glass quality.
The conventional method to make chalcogenide glasses, including rare-earth doped chalcogenide glasses uses a high temperature quenching process that results in a large meniscus, which yields a small volume of useable glass. Moreover, there are refractive index perturbations in the glass that limit the quality of the glass and fiber made from this glass. Optical fibers made from these glasses will cost more because the glass yield is low, and refractive index perturbations will limit their optical performance.